The present invention relates to a control circuit of a MOSFET used for rectifying or circulating an output current of a switching power supply or the like.
FIG. 16 depicts a conventional synchronous rectifying circuit in which a diode rectifying circuit is connected to a secondary side of a forward converter. FIG. 17 is a timing chart illustrating an operation of the circuit shown in FIG. 16. In FIG. 16, reference numeral 101 denotes a DC power supply, 102 denotes a MOSFET (n-channel depletion MOSFET), 103 denotes a transformer, 104, 108 and 109 denote diodes, 105 denotes a control circuit of the MOSFET 102, 106 denotes a smoothing reactor, 107 denotes a smoothing capacitor, and N1, N2 and N3 denote a primary winding, a secondary winding and a tertiary winding, respectively, (number of turns thereof are also taken as N1, N2 and N3, respectively) of the transformer 103. A load, not shown, is conned across the smoothing capacitor 107.
In FIG. 16 and FIG. 17, the MOSFET 102 is subjected to on-off control by the control circuit 105 so that an output voltage becomes constant. When the MOSFET 102 is turned ON in a period (1) in FIG. 17, a DC power supply voltage Vin is applied to the primary winding N1 of the transformer 103. In the secondary winding N2 of the transformer 103, a voltage of (N2/N1) times a primary winding voltage VP1 is generated, which, while storing energy in the smoothing reactor 106 through the diode 108, releases the energy to a load side. An exciting current Im1 flows in exciting inductance (not shown) of the transformer 103.
When the MOSFET 102 is turned OFF in a period (2) in FIG. 17, the exciting energy, being stored in the exciting inductance of the transformer 103, is released from the tertiary winding N3 of the transformer 103 to the DC power supply 101 through the diode 104. In the secondary winding N2 of the transformer 103, a voltage of xe2x88x92(N2/N1) times to a primary winding voltage is generated, and the reverse voltage is applied to the diode 108, which transfers a current ID1, having flowed in the diode 108, into the diode 109. At this time, the energy stored in the smoothing reactor 106 is released to the load side through the diode 109.
In a period (3) in FIG. 17, when the exciting current Im1 becomes zero, a reverse voltage Vin is applied to the diode 104 to cut it off, which causes the primary winding voltage VP1 of the transformer 103 to become zero. In the period (3), the energy stored in the smoothing reactor 106 is continuously released to the load side through the diode 109.
Subsequent to this, in the period (1), the MOSFET 102 is turned ON again and a voltage of (N2/N1) times a primary winding voltage VP1 is generated in the secondary winding N2 of the transformer 103, applying a reverse voltage to the diode 109, which transfers a current ID2, having flowed in the diode 109, into the diode 108.
Thereafter, the period (1) to the period (3) are repeated, by which a waveform of a current IL flowing in the smoothing reactor 106 becomes a synthesized waveform of ID1 and ID2.
FIG. 18 is a second conventional synchronous rectifying circuit in which MOSFETs (n-channel depletion MOSFETs) are used in the forward converter in FIG. 16 instead of the diodes 108 and 109. FIG. 19 is a timing chart illustrating an operation of the circuit shown in FIG. 18.
In FIG. 18, reference numerals 110 and 111 denote MOSFETs, 113 and 114 denote resistors each being connected between a gate of each MOSFET and each end of a secondary winding N2 of the transformer 103. In FIG. 18, components having the same functions as those in FIG. 16 are denoted by the same reference numerals and signs with explanation thereof being omitted.
When an output voltage of a synchronous rectifying circuit using diodes as that in FIG. 16 is a voltage as low as being on the order from 3.3V to 5V, a forward voltage drop of the diode (on the order of 0.5 to 1V) causes a proportion of a conduction loss to become very large.
In a MOSFET with a negative drain current, the drain current flows in a body diode of the MOSFET when no voltage is applied between the gate and the source. This causes a voltage drop on the order of 0.5V. The voltage drop, however, can be reduced by applying a positive voltage between the gate and the source which makes resistivity equivalent to that of the on-resistance to be exhibited. The prior art in FIG. 18 is presented by noting this point.
The differences between the circuits of FIGS. 18 and 16 are shown in FIG. 19. In the period (1), a voltage VQ3, applied between a drain and a source of the MOSFET 111, is applied to the MOSFET 110 as a gate signal for generating a negative drain current IQ2 to reduce the conduction loss of the MOSFET 110. In the period (2), a voltage VQ2, applied between a drain and a source of the MOSFET 110, is applied to the MOSFET 111 as a gate signal for flowing a negative drain current IQ3 to reduce the conduction loss of the MOSFET 111. Hatched portions in IQ2 and IQ3 in FIG. 19 represent periods in which the conduction losses are reduced.
With the prior art as shown in FIG. 18, during the period (3) shown in FIG. 19, a period appears during which no gate voltage is applied to the MOSFET 111 to reduce conduction loss. As a result, device efficiency is decreased and the cooling capacity against heat generation must be increased by enlarging a cooling device. Consequently, the entire device cannot readily be made compact and lightweight.
Accordingly, it is a subject of the present invention to provide a control circuit of a MOSFET for synchronous rectification in which a gate voltage is applied to the MOSFET in almost all of a period in which a current flows in a MOSFET, thereby reducing conduction loss and increasing device efficiency in a device that can be compact and light in weight.
In a preferred embodiment, a cathode of a first diode is connected to a drain of a MOSFET for synchronous rectification, a first current supplying unit is connected to an anode of the first diode, and a resistor is connected between the anode of the first diode and a source of the MOSFET to measure a voltage across the resistor. The voltage across the resistor varies depending on a voltage drop when a current flows in the MOSFET for synchronous rectification. Therefore the value of the voltage across the resistor is compared to a first reference voltage by a voltage comparing unit and the output is amplified. A gate voltage is applied between a gate and a source of the MOSFET for synchronous rectification by a gate driving unit.
Thus, by setting the current level taken as the reference to be small, it becomes possible to apply a gate voltage in almost all of a period in which a current flows in the MOSFET for synchronous rectification, which makes it possible to reduce a conduction loss more than in the prior art shown in FIG. 18.
In a further embodiment, as the above-described first reference voltage, a forward voltage drop in a second diode to which a current is supplied from a second current supplying unit is used. This makes it possible to compensate temperature to forward voltage characteristics of the first diode to enhance a current detection accuracy.
Moreover, by making the first diode and the second diode have forward temperature characteristics the current of the forward direction to temperature-voltage characteristics approximately identical with each other, the current detection accuracy can be further enhanced.
Still further, when a difference between the voltage across the above-described resistor and the first reference voltage becomes equal to or less than a certain value, a gate voltage for the MOSFET is made so as not to be generated. Namely, when a negative current flowing in the MOSFET for synchronous rectification is reduced and the voltage across the above described resistor exceeds the first reference voltage, the gate of the MOSFET for synchronous rectification is to be made OFF. At this time, a current is to be made flow in a body diode of the MOSFET for synchronous rectification to increase the forward voltage drop, which is judged by the first voltage comparing unit as an increase in the current and the gate of the MOSFET for synchronous rectification is brought to be made ON again. As a result, the ON and OFF are repeated to increase a driving loss of the MOSFET. Therefore, the gate of the MOSFET, once being made OFF, is made so as not to be driven until the MOSFET for synchronous rectification is brought into an OFF state. This can suppress the increase in the driving loss
Still further, a saturable reactor may be connected between the drain of the MOSFET and the cathode of the first diode. This reduces a reverse recovery loss when a current-flowing in the body diode of the MOSFET for synchronous rectification is brought into reverse recovery. Along with this, should a zero-crossing of the negative current flowing in the drain of the MOSFET for synchronous rectification occurs before a gate voltage for the MOSFET is brought to a level in which the MOSFET is OFF due to a delay of the control circuit of the MOSFET for synchronous rectification to cause a large current to flow in the positive direction in the drain before the MOSFET is turned off, a rate of increase in the current is limited low after the zero-crossing of the current to make it possible to reduce a turn-off loss.
In addition, the above-described control circuit is made up into an IC chip to be mounted on a chip of the MOSFET, which reduces the number of required components and decreases component mounting area to allow for a compact device.
Still further, a magnetic material having a saturable characteristic is preferably arranged around the chip of the MOSFET for synchronous rectification.